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From Jonathan Ellis <jbel...@gmail.com>
Subject Re: Cassandra benchmark shows OK throughput but high read latency (> 100ms)?
Date Mon, 15 Feb 2010 02:22:12 GMT
are you i/o bound?  what is your on-disk data set size?  what does
iostats tell you?
http://spyced.blogspot.com/2010/01/linux-performance-basics.html

do you have a lot of pending compactions?  (tpstats will tell you)

have you increased KeysCachedFraction?

On Sun, Feb 14, 2010 at 8:18 PM, Weijun Li <weijunli@gmail.com> wrote:
> Hello,
>
>
>
> I saw some Cassandra benchmark reports mentioning read latency that is less
> than 50ms or even 30ms. But my benchmark with 0.5 doesn’t seem to support
> that. Here’s my settings:
>
>
>
> Nodes: 2 machines. 2x2.5GHZ Xeon Quad Core (thus 8 cores), 8GB RAM
>
> ReplicationFactor=2 Partitioner=Random
>
> JVM Xmx: 4GB
>
> Memory table size: 512MB (haven’t figured out how to enable binary memtable
> so I set both memtable number to 512mb)
>
> Flushing threads: 2-4
>
> Payload: ~1000 bytes, 3 columns in one CF.
>
> Read/write time measure: get startTime right before each Java thrift call,
> transport objects are pre-created upon creation of each thread.
>
>
>
> The result shows that total write throughput is around 2000/sec (for 2 nodes
> in the cluster) which is not bad, and read throughput is just around
> 750/sec. However for each thread the average read latency is more than
> 100ms. I’m running 100 threads for the testing and each thread randomly pick
> a node for thrift call. So the read/sec of each thread is just around 7.5,
> meaning duration of each thrift call is 1000/7.5=133ms. Without replication
> the cluster write throughput is around 3300/s, and read throughput is around
> 1400/s, so the read latency is still around 70ms without replication.
>
>
>
> Is there anything wrong in my benchmark test? How can I achieve a reasonable
> read latency (< 30ms)?
>
>
>
> Thanks,
>
> -Weijun
>
>
>
>

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